Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. A summary of common types of photothermographic element constructions is provided by Research Disclosure, Vol. 170, June 1978, Item No. 17029. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. Many of these photothermographic element constructions are also useful as thermographic elements. Most photothermographic elements that rely on photosensitive silver halide to catalyze an image-forming oxidation-reduction reaction can be used as thermographic elements. When use exclusively as a thermographic element is contemplated, a common modification is to omit the photosensitive silver halide and to rely on the imagewise application of heat to drive the image-forming oxidation-reduction reaction, as illustrated by Grant U.S. Pat. No 3,080,254.
It is common practice to include a surface coating in the construction of a thermally processable element. For example, a surface coating can take the form of a transparent coating overlying one or more image-forming layers. Additionally or alternatively, the surface coating can be located to form the back surface of the element on the side of the support opposite the image layer.
In addition to the variety of functions that surface coatings are recognized to perform in imaging elements generally, such as adhesion to the underlying portion (i.e., layer or support) of the element, optical transparency as required (including resistance to fingerprints and scratches), low transport friction, low self-adhesion (necessary for use of spool wound or stacked sheet elements), and sensitometric compatibility with the imaging layers, the surface coatings of thermally processed elements are also commonly relied upon to resist deformation during thermal processing and, to reduce or prevent loss of volatile components during thermal processing. Also, unlike imaging elements that rely on penetration by aqueous processing solutions, the surface coatings of thermally processable elements need not be water permeable and often serve their purpose better when relatively impermeable. In thermally processable elements imaging layer overcoats are often referred to as barrier layers. As a result of differing functional requirements, most conventional selections of surface coatings for thermally processable elements have taken specialized forms unsuited for imaging elements generally.
Research Disclosure, Item No. 17029, cited above, XI Overcoat Layers, catalogues known overcoat components of photothermographic elements.
These subsequent patents illustrate further developments in the art:
Przezdziecki U.S. Pat. Nos. 4,741,992 and 4,828,971 teach the use of polysilicic acid in a surface coating of a thermally processable element. The polysilicic acid is taught to be useful with compatible water soluble hydroxyl containing monomers and polymers.
Przezdziecki U.S. Pat. No. 4,886,739 further teaches incorporation in the imaging layer of at least one hydrolyzed polyalkoxysilane--that is, hydrolyzed Si(OR.sub.1).sub.4 or hydrolyzed R.sub.2 --Si(OR.sub.3).sub.3, to increase image density. These addenda in a hydrophobic imaging layer are further taught to enable increased adhesion of the imaging layer to a hydrophilic overcoat. R.sub.2 is described as a substituted or unsubstituted alkyl or phenyl substituent. To further promote layer adhesion the use of a hydrolyzed polyalkoxysilane in an overcoat layer overlying the imaging layer is optional. Thus, hydrolyzed polyalkoxysilane is contemplated to be present in the imaging layer or both the imaging layer and an overcoat.
Markin et al U.S. Pat. No. 5,310,640 teaches incorporation of polysilicic acid as disclosed in Przezdziecki U.S. Pat. No. 4,741,992 in an overcoat for a limited resistivity (antistatic) backing layer of a thermally processable element.
To prevent self-adhesion (commonly referred to as blocking) of spooled or stacked thermally processable elements, it is common practice to incorporate matting particles. The surface protrusions created by the matting particles create spatial separations between the surfaces of adjacent elements to reduce blocking. Matting particles, also referred to as matting agents or fillers, are disclosed, for example, in Research Disclosure Item No. 17029, XI. Overcoat layers; Przezdziecki U.S. Pat. No. 4,828,971; Mack et al U.S. Pat. No. 5,198,406; Melpolder et al U.S. Pat. No. 5,547,821; Kub U.S. Pat. No. 5,468,603; and Bjork et al U.S. Pat. No. 5,578,548.
Transport of thermally processable elements can also be facilitated by reducing their surface friction independent of the presence or absence of matting particles. This is, however, by comparison infrequently discussed. For example, none of the citations above, except Bjork et al, identify any ingredient, except matting particles, as being introduced for the purpose of facilitating element transport. Bjork et al suggests the optional use of siloxane diamine as a "slip agent" in the topcoat of a thermographic element.